In the manufacture or assembly of a vehicle body, it is known to have an assembly line including a plurality of stations in which the vehicle body is assembled and forwarded as the assembly work proceeds. Typically, the major structural components of the vehicle body are formed of low carbon steel or steel-based alloy panels that are welded together in general welders (i.e., stations wherein primary welding operations are performed to connect the vehicle body components to create the body-in-white body).
The substitution of aluminum or aluminum-based alloy roof components for the low-carbon steel or steel alloy roof components most commonly used in motor vehicles is an attractive option for vehicle mass reduction. Often, however, the remainder of the vehicle body component continues to be fabricated of steel. Joining an aluminum roof component to a steel body component presents difficulties because the aluminum roof component cannot be set and joined in the existing general welder due to equipment and process constraints. Therefore, it is known for manufacturers to secure the aluminum roof component to the steel body component after the weld process in assembly. This process typically includes an adhesive bonding operation and fasteners can also be used to secure the aluminum roof component to the steel body. This approach, though appealing from a vehicle mass-reduction viewpoint, raises issues due to the significantly different coefficients of thermal expansion of aluminum and steel (about 22.5×10−6 m/m K for aluminum and about 13×10−6 m/m K for steel). The combination of the aluminum roof component attached to the steel body component may create compressive stresses in the aluminum roof component when the body component is subjected to elevated temperatures such as those required to cure or bake the paint applied to the steel body component and to cure the adhesive bonding the aluminum roof component to the steel body. These stresses may lead to unacceptable appearance features in the visible segment of the aluminum roof component, and if left unconstrained, the aluminum roof component would bow enough to break the adhesive bond between the aluminum roof component and the steel vehicle body.
One manner of ensuring proper adhesion of the aluminum roof component to the steel body component is to apply the adhesive after completing the electrocoating bath and painting processes. The aluminum roof component is positioned above the steel body component using temporary stand-off fixtures. The spacing created by these stand-off fixtures allows for e-coat and paint coverage. Because the stand-off fixtures do not strongly affix the aluminum roof component to the steel body component, deformation of the aluminum roof component in the ovens is avoided. The aluminum roof component is then removed from the stand-off fixtures, the stand-off fixtures are removed, the adhesive is applied to the aluminum roof component, and the aluminum roof component is positioned on the steel body component in the final installation configuration. Although this manufacturing process can prevent the deformation of the aluminum roof component, assembly time is increased as the stand-off fixtures must first be installed and removed. In addition, because the aluminum roof component is positioned much farther apart from the steel body, the flange of steel body component where the adhesive is applied is exposed to paint and may not provide a good bonding surface unless the flange is masked. However, applying and removing masking to the adhesive bonding surfaces further increases the assembly time and cost.